Nonwoven fabric formed from ultrafine fibers has been employed for middle efficiency or high efficiency air filter material. In particularly, the nonwoven fabric can be so-called melt blown nonwoven fabric. Since the melt blown nonwoven fabric is consisted from ultrafine fibers, tensile strength or tear strength is poor and the fabric is difficult to treat. Accordingly, the melt blown fabric is laminated with another nonwoven fabric composed of filaments or fibers having high fineness, such as spun bonded nonwoven fabric or nonwoven fabric of staple fibers obtained by card method (Patent Literature 1).
In the technique of Patent Literature 1, laminating and integrating a melt blown nonwoven fabric with a spun bond nonwoven fabric or the like is conducted by heat compression. In particular, a sheath-core conjugate filament composed of a high melting point polyester as a core component and a low melting point polyester as a sheath component is used as a constituent fiber of the spun bond nonwoven fabric, and the spun bonded nonwoven fabric is laminated with a melt blown nonwoven fabric and passed through a pair of heat embossed rolls to press and heat the laminated fabrics, wherein the low melting point polyester component is used as an adhesive agent and molten and solidified to laminate and integrate the melt blown nonwoven fabric with the spun bond nonwoven fabric. However, since a portion of the constituent filaments is molten, some openings present in the spun bond nonwoven fabric are filled with the molten low melting point polyester, thus reducing the air permeability of the obtained air filter material.
In order to prevent the reduction of the air permeability, adhesive component is changed from the portion of the constituent filaments of the spun bond nonwoven fabric to the other adhesive agent to bond and to integrate the melt blown nonwoven fabric with the spun bond nonwoven fabric. For example, the other adhesive agent may include emulsion type adhesive agent or hot melt adhesive agent. However, even if such adhesive agents are employed, the adhesive agent might be flown out and filled in the openings of the spun bond nonwoven fabric, thus reducing air permeability.